Message 2001-09-0010: Re: Apomorphy-based definitions

Sat, 25 Aug 2001 14:08:45 -0400 (EDT)

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Date: Sat, 25 Aug 2001 14:08:45 -0400 (EDT)
From: "T. Mike Keesey" <>
To: Nathan Wilson <>
Cc: PhyloCode mailing list <>
Subject: Re: Apomorphy-based definitions

To preface this, I'd like to say that I don't doubt that I may be wrong
(there has to be a reason PhyloCode uses species as the lowest unit, and
not individuals), but I need convincing.

On Fri, 24 Aug 2001, Nathan Wilson wrote:

> On Fri, 24 Aug 2001, T. Mike Keesey wrote:
> > Perhaps I'm missing something (and this is a difficult thing to
> > conceptualize), but can't node and stem definitions really be taken down
> > to the level of the individual?
> >
> > Node: individual organisms A and B have a most recent common ancestral
> > individual C, the latest-occurring individual which is ancestral to both A
> > and B. The node-based clade consists of C and all of its descendants. (For
> > sexual organisms, C might be a breeding pair instead of an individual --
> > not necessarily, though, in non-monogamous organisms.)
> The problem is that "the most recent common ancestral individual" is not
> well defined.  Consider the case of two cousins.  To start with they have
> at least the breeding pair you mention.  However there is also the
> potential for siblings to marry siblings.  In that case you end up with
> four most recent common ancestors.


(monospace fonts on, everyone)

      |   |   |
        |   |
        F   G

(A and B give rise to the siblings C, D, and E. C and D mate and give rise
to F, D and E ate and give rise to G.)

What is the most recent common ancestor of F and G (if they are even
viable in the first place :)? It is D.

Just to try something else, suppose A mates with E and gives rise to H.
The most recent common ancestral individual of F (or C or D or G) and H?
It is A.

There might be some case were it falls apart, but I can't seem to find it.

> You sort of get around this when you
> talk about species since in theory species are only spawned by a single
> parent so a strict hierarchy is created.  However, in cases where species
> are created by hybridizing two otherwise distinct species the node-based
> clade definition becomes undefined.

I still don't see how this is any different. The will still be one or two
most recent common ancestral individuals, as far as I can see.

> I made this argument a while back on this list.  In my opinion the
> node-based clade definition should be changed and based on the set of most
> recent common ancestral individuals. While technically this is not a
> 'clade', any clade that is well defined using the node-based clade
> definition includes the same individuals with the new definition. In
> addition, this definition handles hybrids with no problem and as you point
> out can be used at the sub-species level.  I came up with a precise graph
> theoretic definition which I can dig up for you if you're interested.

I, for one, would be quite interested.

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